This invention relates to a method for charging a nickel-metal hydride battery that includes a non-porous, alkali ion conducting separator.
Nickel-metal hydride storage batteries are widely used for the power sources of cordless electronic equipment, power tools, electric vehicles and the like. Conventional nickel-metal hydride batteries are composed of a positive electrode containing nickel hydroxide, a negative electrode containing a hydrogen-absorbing metal alloy, a separator interposed between the positive and negative electrodes, and an electrolyte. FIG. 1 shows a schematic representation of typical nickel metal hydride battery 10 having a positive electrode 12, a negative electrode 14, and a separator 16.
The nickel hydroxide positive electrodes are similar in design to conventional nickel-cadmium electrodes. Pasted and sintered-type positive electrodes have proven to be economical and rugged, exhibiting excellent high-rate performance, long cycle life, and good capacity.
The hydrogen-absorbing metal alloys used in the negative electrode were developed in the 1970s from research on the storage of hydrogen for use as an alternative energy source. Some metallic alloys were observed to form hydrides that could capture (and release) hydrogen in volumes up to nearly a thousand times their own volume. By careful selection of the alloy constituents and proportions, the thermodynamics could be balanced to permit the absorption and release process to proceed at room temperatures and pressures. In such alloys, the small hydrogen atom is absorbed into the interstices of a bimetallic alloy crystal structure.
The separator provides electrical isolation between the electrodes while still allowing efficient ionic transport between them. The separators used in nickel metal hydride cells are often similar to those used in nickel-cadmium cells and typically include woven or nonwoven fabric comprising a polyamide and polyolefin, or a porous film of a fluorine plastic film. A known effective separator is a nylon fiber blend. Other polymeric fibers are used in separators. a woven or nonwoven is generally used
The electrolyte used in the nickel-metal hydride cell is alkaline. It commonly includes an aqueous solution of potassium hydroxide. The electrolyte may contain other minor constituents to enhance cell performance.
The charge and discharge reactions for nickel-metal hydride batteries are shown below:Positive electrode: Ni(OH)2+OH−→NiOOH+H2O+e− (charge)NiOOH+H2O+e−→Ni(OH)2+OH− (discharge)Negative electrode: M+H2O+e−→MHab+OH− (charge)MHab+OH−→M+H2O+e− (discharge)Overall reaction: Ni(OH)2+M→NiOOH+MHab (charge)NiOOH+MHab→Ni(OH)2+M (discharge)
Where M is a hydrogen absorbing alloy and Hab is absorbed hydrogen. From the overall reactions shown above, hydrogen moves from the positive to negative electrode during charge and reverses direction during discharge, with the electrolyte taking no part in the reaction.
Self-discharge is a phenomenon in essentially all rechargeable batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes. Self-discharge decreases the shelf-life of batteries and causes them to have less charge than expected when actually put to use. How fast self-discharge in a battery occurs is dependent on the type of battery and temperature. Nickel-based batteries are significantly affected by self-discharge (nickel cadmium, 15-20% per month; nickel metal hydride, 30% per month). Self-discharge is a chemical reaction and tends to occur more quickly at higher temperatures. Storing batteries at lower temperatures may reduce the rate of self-discharge and preserve the initial energy stored in the battery.
Without being bound by theory, it is believed the self-discharge problem associate with nickel metal hydride batteries is a result of hydrogen passing through the porous separator.
It would be an improvement in the art to provide a nickel metal hydride battery with reduced or limited self-discharge.